1. Field of the Invention
The present invention relates to a hexagonal ferrite magnetic powder and to a method of manufacturing the same, and more particularly, to a microparticulate hexagonal ferrite magnetic powder with high coercive force and a low plate ratio, and to a method of manufacturing the same.
The present invention further relates to a magnetic recording medium employing the above hexagonal ferrite magnetic powder, and to a method of manufacturing the same.
2. Discussion of the Background
Recently, ferromagnetic metal powders have come to be primarily employed in the magnetic layers of magnetic recording media for high-density recording. Ferromagnetic metal powders are comprised of acicular particles of mainly iron, and are employed in magnetic recording media for various applications in which minute particle size and high coercive force are required for high-density recording.
With the increase in the quantity of information being recorded, magnetic recording media are required to achieve ever higher recording densities. However, in improving the ferromagnetic metal powder to achieve higher density recording, limits have begun to appear. By contrast, hexagonal ferrite magnetic powders have a coercive force that is high enough for use in permanently magnetic materials. Magnetic anisotropy, which is the basis of coercive force, derives from a crystalline structure. Thus, high coercive force can be maintained even when the particle size is reduced. Further, magnetic recording media employing hexagonal ferrite magnetic powder in the magnetic layers thereof can afford good high-density characteristics due to their vertical components. Thus, hexagonal ferrite magnetic powder is an optimal ferromagnetic material for achieving high density. Hexagonal ferrite magnetic powders are normally manufactured by a glass crystallization method (see Japanese Unexamined Patent Publication (KOKAI) No. 2006-5299 or English language family member US2005/0282043A1 and Japanese Unexamined Patent Publication (KOKAI) No. 2002-313619), a coprecipitation method (see Japanese Examined Patent Publication (KOKOKU) Heisei No. 4-32005), and the like. The contents of the above applications are expressly incorporated herein by reference in their entirety.
A high S/N ratio is required for increase of the recording density of a magnetic recording medium. Setting the coercive force Hc high to inhibit recording demagnetization or self-demagnetization when recording at short wavelengths and designing the particles of magnetic powder to be as small as possible to inhibit noise are known to be effective ways to raise the S/N ratio. However, conventionally employed manufacturing methods, such as the methods described in the above applications, tend to increase the plate ratio when reducing the plate diameter. The quantity of elements substituting for a portion of Fe is reduced to raise the Hc of hexagonal ferrite. Although an increase in Hc can be achieved by simply reducing the quantity of the elements that substitute Fe, the plate ratio of the particles increases. Reduction of the filling density of hexagonal ferrite particles and aggregation of the particles, known as stacking, occur in the magnetic layer of the magnetic recording medium when the plate ratio increases. This results in an increase in noise, making it difficult to achieve adequate performance as a magnetic recording medium for high-density recording.
By contrast, Japanese Unexamined Patent Publication (KOKAI) No. 2007-91517, which is expressly incorporated herein by reference in its entirety, describes the forming of a coprecipitate comprised of a desired hexagonal magnetoplumbite-type ferrite by the reverse micelle method, and the use of this coprecipitate to obtain hexagonal magnetoplumbite-type ferrite.
The hexagonal ferrite obtained by the method described in Japanese Unexamined Patent Publication (KOKAI) No. 2007-91517 has both a low average plate diameter and a low plate ratio, as well as a high Hc. Thus, the use of this hexagonal ferrite can yield a magnetic recording medium with a higher S/N ratio than magnetic recording media in which hexagonal ferrite obtained by conventional manufacturing methods is employed. However, as the recording density is further increased, there is need for an even higher S/N ratio, exceeding the S/N ratio that can be achieved by the technique described in Japanese Unexamined Patent Publication (KOKAI) No. 2007-91517.